The invention relates to the band expansion of speech for telephones, in particular for mobile telephones.
FIG. 1 of the accompanying drawings is an exemplary illustration of a wide-band speech signal having a bandwidth of around 8 kHz. Although most of the information carried by the speech signal is contained in components of the speech signal having frequencies up to 4 kHz, as can be seen clearly from the diagram, nevertheless significant information is contained in components of the speech signal having frequencies in the range approximately 4-8 kHz.
An exemplary illustration of an equivalent narrowband speech signal having a bandwidth of around 4 kHz is also shown in FIG. 1.
The bandwidth of speech carried by the existing telephone system infrastructure is generally limited to around 4 kHz. Although speech signals having a bandwidth of 4 kHz are intelligible, the loss of the higher frequencies from the speech signal results in the speech produced by telephones sounding unnatural.
Many suggestions have been made previously to enhance the quality of speech signals in telephone systems by bandwidth expansion of the narrowband speech signal.
One conventional way of creating a wide-band speech signal from a narrowband speech signal relies on the characteristics of speech and uses pitch periodicity and the spectral envelope of the narrowband speech signal to estimate the pitch periodicity and the spectral envelope of the missing wide-band signals frequencies.
However, algorithms which estimate the pitch periodicity and the spectral envelope of the missing wide-band signals frequencies tend to introduce unwanted artefacts which reduce speech quality.
Spectrum expansion methods that utilise aliasing effects resulting from sampling rate conversion and subsequent digital filtering for spectrum shaping have also previously been proposed.
One example of this technique shows a narrowband speech signal sampled at 8 kHz is expanded by an interpolator with 16 kHz sampling. The resulting signal is fed to two parallel filter paths. In the first filter path the interpolated signal is filtered with a low pass filter to obtain the original input signal. In the second filter path the interpolated signal is filtered with a shaping filter to generate a signal in the frequency range 4-7 kHz. The signals resulting from the two parallel filter paths are then level adjusted and added together to obtain the desired wide-band signal.
However, although the circuit configuration used in this method is relatively simple when compared with the previously used methods based on estimates of the spectral envelope and periodicity of the speech signal, the method set out in this paper still involves extensive filtering and requires level adjustment of the signals in the different filter paths prior to the summation of the filtered samples from each path to obtain the wide-band output speech signal.
The prior art proposals to expand speech bandwidth for telephones have the drawback that they are fairly complex and computationally intensive. In addition prior art proposals which seek to estimate the higher band frequencies can introduce unwanted artefacts into the signal, therefore degrading the speech quality.
The present invention seeks to provide a method of expanding the speech bandwidth for telephones which provides improved speech quality when compared with the narrowband speech signal.
Embodiments of the method in accordance with the invention have the advantage that they can be implemented with low complexity.